1. Field of the Invention
The invention relates to a novel structure for a laser utilizing a gaseous active medium which reduces sputtering effects, optimizes efficiency and utilizes air cooling.
2. Description of Related Art
Ion lasers, in general, are of a type which utilize a gaseous active medium and emit an output in both the visible and ultraviolet regions of the spectrum. Three types of gasses which may be used as active media in ion lasers are Argon (Ar), Krypton (Kr) and Xenon (Xe). In typical applications, pure gas is used with normal operating pressure slightly under 1 torr.
Typically, ion lasers include discharge tubes which are excited by a high-current discharge that passes along the length of the tube and is concentrated in a small-diameter bore, or at the center of the discharge tube. The most common means of accomplishing excitation of the active medium is through arrangement of an anode and cathode placed at opposing ends of the laser bore. A high-current discharge in the bore is formed upon application of a potential between the anode and cathode. Typically, an initial voltage spike of a few thousand volts is required to ionize the gas, after which the voltage may be maintained at a level in the range of, for example, approximately 90 to 600 volts with the discharge current at approximately 10-70 amps.
Several problems arise from typical anode/cathode methods of exciting the gaseous active medium in ion lasers. Specifically, strong electron flow in the discharge current tends to push neutral atoms toward the positively charged anode, while ions migrate towards the negative cathode. This migration creates a need for gas circulation in the tube to ensure uniform excitation. Further, upon excitation, the ionized plasma contained in the laser tube reaches high temperatures causing a sputtering effect which erodes both the laser bore and the anode and cathode. The sputtering also leads to entrapment of the gas, which requires a supply of extra gas to replenish gas depleted during operation. Typically, ion lasers use a magnetic field parallel to the axis of the laser bore to concentrate the discharge current to the center of the bore.
The intense heat generated in exciting the gas necessitates cooling of the laser bore. Both air-cooled and water-cooled methods of cooling have been utilized in ion lasers. Water-cooling ion lasers generally involves arranging water circulation along the outer surface of the bore. Such structures generally increase the complexity and bulk of the laser. Air-cooled lasers are of a variety of types generally using forced air circulation and some form of convection cooling structure about the bore to provide heat dissipation.
Other methods of exciting ion lasers have been utilized in the prior art. One such method of exciting the active medium is through the use of RF energy coupled by means of a waveguide to the laser bore. An RF excited laser structure is shown in U.S. Pat. No. 3,521,119 to Ahmed et al. wherein the RF energy is coupled by means of a waveguide and coupling coils, the waveguide comprising a 2-plate transmission line with the two plates tapered towards each other. Coupled to the transmission line is a plurality of coils oriented in a collinear serial relationship having their respective opposite ends coupled to the first and second plates. RF energy is supplied to the distal ends of the waveguide plates and travels down the transmission line, reaching each of the taps in successive order.
Another RF excitation structure is shown in U.S. Pat. No. 4,513,424 wherein X-band microwaves are coupled to the active medium through means of a coupling plate forming a common wall between a waveguide and an RF cavity to produce a standing wave pattern in the waveguide which excites the active medium.
Yet another RF induction method for exciting an active laser medium was used in the Spectra Physics Model SP 141 laser which utilized a tube-type RF source to excite the laser active medium. More particularly, a helical coupling coil was provided about the plasma tube to couple RF energy to excite the active medium. The SP 141 also included a closed-loop cooling structure arranged around the helical coupling coil and mineral-free water was pumped through the tubular cooling structure to cool the laser bore.